Method for creating an image from a 3d volume

ABSTRACT

The invention relates to a method for creating a virtual dental image from a 3D volume ( 1 ) comprising volumetric image data. Firstly, a sub-volume ( 8, 12, 15, 18 ) of the 3D volume ( 1 ) is defined and then a virtual projection image ( 30, 41 ) is generated for said sub-volume ( 8, 12, 15, 18 ) from a specific X-ray imaging direction ( 11 ) by computation of the volumetric image data in said X-ray imaging direction ( 11 ).

TECHNICAL FIELD

The invention relates to a method for creating an image from a 3D volumecomprising volumetric image data of a mandibular arch and of teeth.

PRIOR ART

Several methods for creating a virtual three-dimensional X-ray image aredisclosed in the prior art. In the method of digital volume tomography(DVT), an X-ray source assembly and a sensor interconnected by an axisare moved around the head of the patient in a defined plane. Projectionimages are generated from various positions along this circular path,and in the next step a three-dimensional volume is computed from saidprojection images. The 3D volume can then be displayed by applicationsoftware on a monitor. In the method of computed tomography (CT), theimage is formed on the basis of continuous imaging of the projectionfrom different directions. Usually, the computed 3D reconstructions arecomposed of individual slices according to the sliced imaging technique,which slices extend through the object at right angles thereto. In thisway, the X-ray absorption value can be defined for each of the volumeelements of the object, the so-called voxels.

DE 101 08 295 A1 discloses a method for identifying objects,particularly teeth, and a system for effecting the same, based on adigitized X-ray image, in which method regions in the object aredelimited by segmentation and/or edge detection using image-processingalgorithms, and these regions are associated by computation withparameters of the X-ray apparatus and of the patient, where appropriate,to give further parameters. Furthermore, a method is disclosed in whichthe objects are manually or automatically specified, and, in one step,the object is selected for which more information is to be saved,retrieved, or deleted, and a reference relating to the object is savedin a further step, which reference makes it possible to specify theinformation to be displayed.

The methods of edge detection and segmentation are applied to the imagedata of the digitized X-ray image with the assistance of a computer, andthe defined edges and segments are grouped by the so-called clusteringtechnique.

U.S. Pat. No. 5,179,579 discloses a method of displaying intraoral X-rayimages. The intraoral X-ray images are produced, digitized, anddisplayed together with an icon of that portion of the anatomy, fromwhich the X-ray image has been taken. The images of the anatomical sitesare displayed with the respective icon in normal anatomical relation toeach other on a monitor. The icon is used by the user in order to selectthe respective X-ray image of the anatomical site. A miniaturized imageof the dentition, of a row of teeth, or of the individual teeth may beused, for example, as the icon.

U.S. Pat. No. 6,190,042 B1 discloses a device for creating improvedintraoral X-ray images. The device comprises a bite block, a guidingrod, an aiming ring, and an additional ring. The bite block comprises afilm holder disposed at right angles to the top surface of the biteblock. The guiding rod is connected to the bite block. The deviceguaranties a predefined distance between the film in the film holder andthe external ring.

A drawback of the devices and methods disclosed in the prior art is thatthe generation of the intraoral image requires an elaborate device to bepositioned and secured in the mouth of the patient, which device mostlycomprises a bite block for securing the device, while an X-ray film or adigital X-ray sensor is disposed in the form of imaging means on thisdevice in the oral cavity of the patient. Positioning of this device isdifficult to carry out on account of the fact that the patient altersthe position of the device relative to the upper jaw while biting on theblock and that the distance of the imaging element from the teeth isunknown.

In the case of a series of several intraoral images, gaps may resultbetween the individual images due to the imprecise positioning of thedevice, and thus the oral cavity of the patient is incompletely imaged.

Furthermore, the imaging volume of the intraoral images with respect tothe teeth in the imaging direction is not known.

DE 10 2008 008 733 A1 discloses a method for creating a tomogram, forwhich purpose, in particular, a dental X-ray panoramic tomogram isproduced from a digital 3D volume showing X-ray absorption values, andX-rays are passed virtually through the 3D volume, as the object to beimaged, by means of a virtual X-ray source, and the resulting virtualimage is recorded by a virtual detector. The virtual X-ray source andthe virtual detector are moved virtually past the object being imaged toform a sharp layer having a blurred region.

The width of the virtual detector, the broadening of the virtual fanbeam and the simulated speed of rotation of the virtual X-ray source andthe virtual detector can be altered in order to control the thicknessand position of the imaged sharp layer. In one embodiment, a definedsub-region, the extent of which corresponds to the sharp layer, isselected from the 3D volume, and X-rays are passed virtually through thesub-region at right angles to the contour of the sub-region to form apanoramic image.

A drawback of this method is that the virtually generated panoramicimage, unlike an intraoral projection image, represents a distortedimage of the entire dental arch and thus makes it difficult to diagnosespecific findings.

It is therefore an object of this invention to generate atwo-dimensional projection image for a defined imaging volume.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a method forcreating a virtual dental image from a 3D volume comprising volumetricimage data. Firstly, a sub-volume is specified from the 3D volume. Avirtual projection image is generated for this sub-volume from a definedX-ray imaging direction by computing the volumetric image data in thisX-ray imaging direction.

The 3D volume can be a three-dimensional X-ray image generated by meansof any desired three-dimensional radiographic procedures such as DVT orCT, and the volumetric image data can be composed of voxels having X-rayabsorption values. The 3D volume can also be imaged by means of otherimaging techniques such as MRI. The 3D volume can include the entiredental arch including the teeth, or a portion thereof. The volumetricimage data of the 3D volume can be composed of voxels, vector elements,or a combination of voxels and vector elements. The volumetric imagedata can also be composed of point clouds containing vector elements. Adefined sub-volume having any desired basal surface and a definedthickness in the X-ray imaging direction is selected from this 3Dvolume. In a further step, a virtual projection image is generated forthis selected sub-volume by computing the volumetric image data such asvoxels or vector elements in the X-ray imaging direction. This resultsin a virtual projection image of the selected sub-volume, which virtualprojection image corresponds to an intraoral X-ray image of a specificregion of the dental arch. The volumetric image data can be computed inthe X-ray imaging direction, for example, by summation of the individualvolumetric image data such as voxels or vector elements disposed onebehind the other in the X-ray imaging direction. The volumetric imagedata can alternatively be computed by means of integration following theapplication of a defined function to the voxels disposed in the X-rayimaging direction.

A defined function can be applied to the volumetric image data prior tocomputation of the volumetric image data in order to weight thevolumetric image data in the X-ray imaging direction in such a way thatspecific portions, such as carious regions, that are of significance tothe evaluation, are highlighted, while less weight is given to otherportions, such as a filling, that are of no significance to theevaluation. Such a function can be applied, for example, to the graytones of the individual elements of the volumetric image data.

The 3D volume can be composed of voxels that represent volume elementsof the object imaged. Any three-dimensional dental image can be used asthe 3D volume, for example a three-dimensional X-ray image, namely adigital volume tomogram or a computed tomogram comprising voxels havingX-ray absorption values in the form of gray tones or a magneticresonance tomogram comprising voxels representing volume elements of thetissue imaged. The 3D volume can alternatively be a three-dimensionalultrasonogram.

When the projection image is generated, the volumetric image data can becomputed by summation of the X-ray absorption values in the X-rayimaging direction with the virtual X-ray imaging direction beingparallel across the entire basal surface or in the form of a conical fanas an extension of virtual X-rays emanating from a virtual X-ray source.

The basal surface may be of any desired shape, for example, a circle, arectangle, or a defined contour of the object to be imaged.

The sub-volume may be of any other basic geometric shape, such as atetrahedron, or it may have the shape of a segmented sub-volume.

An optimal projection image is generated having a basal surface in theform of a rectangle in a parallel X-ray imaging direction so that theselected sub-volume has the shape of a cuboid.

The projection image should have dimensions that are equal to the sizeof a conventional intraoral image created with single-tooth radiographs,namely a width ranging from 15 mm to 30 mm and a height ranging from 25mm to 40 mm.

One advantage is that the patient need not again be exposed to x-rays inorder to generate projection images. The projection images, such asintraoral X-ray images of specific regions, can be generated from theexisting three-dimensional image data of the 3D volume.

A further advantage is that the sub-volume can be defined precisely bythe user with the assistance of a computer and, for example, intraoralX-ray images of individual teeth can be generated.

Yet another advantage is that the sub-volume can be selected such that aregion of interest in the 3D volume such as a filling can be excludedvirtually in order to compute a projection image that does not containthe portion that has been excluded. This makes it possible to diagnose,for example, secondary caries located under the filling. This method ofexcluding specific structures can be applied to fillings, dentin layers,tooth enamel structures, etc., which can be excluded virtually followingappropriate segmentation.

Advantageously, the 3D volume comprising volumetric image data may be athree-dimensional X-ray image having X-ray absorption values.

Thus the virtual projection image corresponds to a two-dimensional X-rayimage created in a defined X-ray imaging direction.

Advantageously, the volumetric image data can be computed by summationof the volumetric image data, such as voxels or vector elements, thatare disposed successively along the X-ray imaging direction.

This makes it possible, for example, to compute each pixel of theprojection image by summation of the voxels in the X-ray imagingdirection with regard to their gray tones.

Advantageously, the volumetric image data may be computed byintegration, following the application of a defined function to theelements of the volumetric image data disposed in the X-ray imagingdirection.

This is a further alternative to computing the volumetric image data andpermits different weighting of the elements according to a definedfunction. This function can be designed such that specific portions areinvisible or only weakly visible, while more weight is given to theportions of diagnostic significance.

Advantageously, the sub-volume may be defined manually or automaticallywith the assistance of a computer.

The sub-volume can be defined manually by a user by the use of inputdevices. The sub-volume can alternatively be defined automatically usinga computer and software for processing image data and using conventionalimage-processing techniques such as the identification or segmentationof defined sub-objects, such as teeth or jawbones.

Advantageously, the sub-volume can be defined automatically withreference to a segmented sub-object, and the external contour of thesub-volume is defined so as to correspond to an external contour of thesegmented sub-object.

A conventional method of pattern recognition can be used forsegmentation purposes. By this means a volume includes only thesub-object to be diagnosed so that the adjoining tissue, such as gumsand the jawbone, is ignored and diagnosis is thus facilitated.

Advantageously, the external contour of the sub-volume can be definedmanually such that it corresponds to the external contour of acharacteristic sub-object.

This makes it possible for the user to define the external contour ofthe sub-volume manually using input devices according to the externalshape of the sub-object to be diagnosed, such as a tooth or a group ofteeth.

Advantageously, the teeth and/or the dental arch can be identified firstfrom the 3D volume. Then the position and orientation of the teethand/or the dental arch in the 3D volume are defined in order to definethe sub-volume and the X-ray imaging direction for the purpose ofgenerating the virtual projection image.

The teeth and the dental arch can be identified from the 3D volumeeither automatically with the assistance of a computer or manually by auser employing input devices, such as a mouse and a keyboard and adisplay device such as a monitor. In the automatic identification orsegmentation, use is made of conventional methods for patternrecognition in which objects are segmented and the relationship betweenthe objects is sought. The method of pattern recognition can include thefollowing steps: preprocessing, feature extraction, feature reduction,and classification of features. During preprocessing, undesirable orinsignificant components of the image data are removed. During featureextraction, defined features are extracted from the image data bycomparing them with known patterns from a database such as a database ofcharacteristic teeth. Automatic comparison is carried out usingtransformation functions and scaling, a comparative factor beingcomputed by calculating the variance between a pattern from the imagedata and an expected pattern from the database. Feature reductioninvolves examination of the features to determine those that areimportant for classification purposes and those that can be omitted. Inparticular, the extracted patterns of teeth and the dental arch are ofsignificance for this method, while the remaining extracted features canbe disregarded. In the last step of the classification, the essentialfeatures recognized, such as teeth and characteristic shapes of thedental arch, are split up into appropriate classes, such as incisors,molar teeth, tooth roots, and jawbones.

Feature extraction involves the use of known methods such as rasteranalysis, cluster analysis, and pattern matching.

Advantageously, the sub-volume can be defined in the form of a prismhaving an arbitrary basal surface and a defined thickness in the X-rayimaging direction.

The sub-volume can be defined automatically with the assistance of acomputer with reference to specific recognized objects such as teeth orgroups of teeth, or it can be input manually by the user. Firstly, abasal surface, and then the thickness in the X-ray imaging direction aredefined. This results in a geometric prism, and the X-ray imagingdirection can extend at right angles to the basal surface.

In this way the sub-volume can be defined such that specific objects,such as teeth or groups of teeth, or specific portions of the jawbone,are disposed in the sub-volume. By defining the thickness, it ispossible to delimit the sub-volume such that it contains only the objectto be imaged, while other objects, such as a patient's cheek disposed infront of or behind the object to be imaged in the X-ray imagingdirection, are not included in the sub-volume and thus do not emerge asdisturbing artifacts in the computed projection image.

Advantageously, the virtual projection image can be saved in the form ofan intraoral image in application software for the administration ofdental X-ray images.

A virtual projection image, created in an X-ray imaging directionoriented lingually or palatally at right angles to the course of thedental arch and having a sub-volume that includes specific teeth, willcorrespond to an intraoral image and can be saved as such in availableapplication software.

This makes it possible to use the computed projection image fordiagnosis in the manner of a conventional intraoral image.

Advantageously, the virtual projection image can be displayed on adisplay device in the form of a virtual intraoral image.

This makes it possible to display the virtual projection image as aconventional intraoral image on a display device, such as a computermonitor, using available application software.

Advantageously, a plurality of projection images can be generated fromdifferent sub-volumes in different X-ray imaging directions according toa predefined schema.

These sub-volumes can be of arbitrary shape. They can have variablethickness and a selected basal surface or they can be defined such thattheir external contour corresponds to a segmented sub-object, such as atooth or a group of teeth. The projection images created canalternatively be displayed in a defined arrangement by means of thedisplay device.

This makes it possible to generate a series of a plurality of virtualintraoral images covering the entire oral cavity of the patient.

The series of virtual intraoral images can be generated according to aconventional schema as shown in FIGS. 5A to 5S of U.S. Pat. No.5,179,579 and disclosed in the relevant portions of the description.

The individual virtual intraoral images of the series can include one ormore teeth. A virtual intraoral image of a plurality of teeth also makesit possible to diagnose regions between the teeth.

This enables intraoral images of the individual teeth or groups of teethto be created in order to improve the diagnosis. Thus a series ofintraoral images of the entire oral cavity of the patient (full mouthseries) can be generated. Unlike conventional intraoral images createdusing a film or an intraoral sensor, it is possible to arrange thevirtual sub-volumes in a precise manner such that the sub-volumes canadjoin each other without any gaps between them in a series of images,which improves possible diagnosis results.

Advantageously, each sub-volume can include image data comprisingvolumetric image data of an individual tooth or a group of teeth in thedental arch in order to create separate projection images of anindividual tooth or of individual groups of teeth.

Thus a series of projection images of the individual teeth in both ofthe dental arches can be generated.

The sub-volume may include only portions of teeth, such as half a tooth.The sub-volume may include a tooth and two tooth gaps.

Advantageously, an X-ray imaging direction can be defined for eachsub-volume in the palatal direction towards the gums or in the lingualdirection towards the tongue, with the individual sub-volumes beingdirectly juxtaposed or partially overlapping each other, so to create aseries of projection images of the entire dental arch.

Thus the X-ray imaging direction of the resulting projection imagescorresponds to the usual orientation of intraoral images, so that theseries formed can be used for diagnosis in the same manner as a seriesof conventional intraoral images.

Advantageously, an X-ray imaging direction can be defined for eachsub-volume in the occlusal direction along a tooth axis, with theindividual sub-volumes being directly juxtaposed or partiallyoverlapping each other, so as to create a series of occlusal projectionimages of the entire dental arch.

This makes it possible to effect diagnosis from the occlusal directionof the teeth over the entire dental arch.

Advantageously, the virtual projection images produced from the 3Dvolume can be arranged automatically so as to include the complete rangeof teeth disposed in the oral cavity.

This makes it possible to generate a series of intraoral images (fullmouth series) so as to include the complete oral cavity of the patient.

Advantageously, the virtual projection image can be equivalent to anintraoral image created by means of an intraoral imaging devicecomprising an X-ray source assembly and an image receptor, such as anintraoral sensor.

In the case of an X-ray imaging direction extending at right angles tothe orientation of the dental arch, the projection image corresponds toa conventional intraoral image of a portion of the dental arch. Thevirtual projection image can alternatively be created for individualteeth in an X-ray imaging direction extending along the dental arch.Such projection images can be advantageous for the diagnosis of specificfindings on the approximal surfaces in an interdental gap.

Advantageously, the 3D volume can be imaged by methods involving digitalvolume tomography (DVT), computed tomography (CT), three-dimensionalultrasonics, or magnetic resonance imaging (MRI).

Thus a 3D volume is generated which shows high resolution and highimaging quality.

Advantageously, specific physical conditions relevant to conventionalintraoral images, such as scattering, the properties of the X-ray sourceassembly, the X-ray spectrum, or the arrangement of the detectorrelative to the X-ray source assembly can be allowed for when creatingthe virtual projection image for the defined sub-volume.

These factors can be simulated, for example, with the assistance of acomputer or retrieved from a database comprising factors for differentarrangements whilst allowing for different physical conditions.

Thus a conventional intraoral image is replicated in terms of thephysical conditions in order to make diagnosis possible in aconventional manner.

Advantageously, specific unwanted sub-objects such as fillings orjawbones can be excluded from the sub-volume when the sub-volume isbeing defined or they can be allowed for with a lower weighting whencomputing the volumetric image data, in order to facilitate examinationof the sub-objects, such as teeth, included in the sub-volume, forexample for secondary caries.

This makes it possible to improve diagnosis of, in particular, secondarycaries disposed in most cases between the filling and the toothsubstance.

Advantageously, defined system characteristics of a conventionalintraoral image, such as the detector efficiency, a detectorcharacteristic or a plurality of detector characteristics, and/or thedetector sensitivity in the case of different detectors, can besimulated for the defined sub-volume when the virtual projection imageis being generated, and the type of detector, such as a memory foilsystem, single-tooth radiographs of variable sensitivity, or a digitalintraoral sensor, can be selected virtually.

These system characteristics can also be retrieved from a database fordifferent image data.

This makes it possible to simulate system characteristics of specifictypes of detector in order to imitate as precisely as possible aconventional intraoral image created with this type of detector. Thetype of detector can be selected by the user virtually using inputdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are shown in the drawings, inwhich:

FIG. 1 is a diagrammatical representation of a 3D volume comprisingselected sub-volumes;

FIG. 2 is a diagrammatical representation of a projection imagegenerated from a sub-volume;

FIG. 3 is a diagrammatical representation of an array of a plurality ofprojection images of the entire oral cavity of a patient;

FIG. 4A is a diagrammatical representation of a sub-volume with parallelrays;

FIG. 4B is a diagrammatical representation of an alternative sub-volumein the form of a conical fan;

FIG. 4C is a diagrammatical representation of an alternative sub-volumewith a trapezoidal basal surface;

FIG. 5 shows a schema including a plurality of selected sub-volumes;

FIG. 6 is a diagrammatical representation of a detail of the 3D volumecomprising a single molar tooth;

FIG. 7 is a diagrammatical representation of an alternative detailcomprising a filling between two molar teeth.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 is a diagrammatical representation of a 3D volume 1 imaged bymeans of digital volume tomography (DVT) or computed tomography (CT).The 3D volume 1 is composed of voxels having specific X-ray absorptionvalues. The 3D volume comprises a plurality of structures such as thecentral incisors 2, the lateral molars 3, and the maxillary arch 4, themandibular arch 5, the upper jawbone 6, and the lower jawbone 7. The 3Dvolume 1 is displayed in application software on a computer monitor (notshown in the figure). The 3D volume can be moved and rotated arbitrarilyin order to alter the viewing direction of the display. In the firststep of the method of the invention, a virtual sub-volume 8 having aspecific basal surface 9 and a specific thickness 10 in an X-ray imagingdirection 11 is defined. The sub-volume can be defined automaticallywith the assistance of a computer or manually by a user. When thesub-volume is defined automatically, the individual teeth are identifiedand classified by means of a conventional method of pattern recognition,and the sub-volume is defined such that specific portions of the dentalarch are included therein. When the sub-volume is defined manually, theuser can first define the base 9 by means of input devices and thendefine the thickness of the sub-volume. The sub-volume 8 defined isdisplayed highlighted in relation to the 3D volume 1 in the applicationsoftware. The second sub-volume 12 having the basal surface 13 and thethickness 14, the third sub-volume 15 having the basal surface 16 andthe thickness 17, and the fourth sub-volume 18 having the basal surface19 and the thickness 20 are defined accordingly. When the sub-volumes 8,12, 15, and 18 are defined, the thickness 10, 14, 17, 20 is oriented inan X-ray imaging direction extending at right angles to the course 21 ofthe lower row of teeth in the palatal direction towards the gums or inthe lingual direction towards the tongue. The individual sub-volumes 8,12, 15, and 18 adjoin each other without gaps therebetween. Atwo-dimensional virtual projection image corresponding to a conventionalintraoral image of the respective portion of the dental arch within thesub-volume is generated from the selected sub-volumes by summation ofthe X-ray absorption values in the X-ray imaging direction in the secondstep of the method of the invention with the assistance of a computerusing an algorithm. The shape of the sub-volumes 8, 12, 15, and 18 isequivalent to that of a prism having a basal surface 9, 13, 16, 19 and athickness 10, 14, 17, and 20 respectively.

The projection image generated is saved in the form of a virtualintraoral image in a database in a conventional software application.

FIG. 2 is a diagrammatical representation of a projection image 30generated from the fourth sub-volume 18 of the molar teeth of themandibular arch 5. The virtual projection image 30 corresponds to aconventional intraoral image and is displayed in the softwareapplication on a computer monitor and saved accordingly in a databaseintended for this purpose.

FIG. 3 is a diagrammatical representation of an array 40 of a pluralityof projection images of the mandibular arch 5 and the maxillary arch 6of the entire oral cavity of the patient illustrated in FIG. 1. Thegenerated projection image 30, as shown in FIG. 2, can be seen bottomleft in FIG. 3. The remaining projection images 41 are generatedaccordingly from the other sub-volumes 8, 12, 15 illustrated in FIG. 1and additional sub-volumes of the maxillary arch 6 that are not shown inFIG. 1. The sub-volumes for generating the projection images 30, 41 asshown in FIG. 3 partially overlap each other. The array 40 of thegenerated intraoral images 30, 41 as shown in FIG. 3 is producedaccording to a predefined schema, namely the so-called “full mouthseries”. The projection images 30, 41 can alternatively be generatedaccording to another, arbitrary schema. It is equally possible togenerate projection images in the direction of the course 21 of thelower jawbone or of the upper jawbone 6 so that the regions between theteeth are imaged on such a projection image.

FIG. 4A shows a sub-volume 18 comprising one molar tooth 3 in itsentirety and parts of two adjacent teeth, the basal surface 19 being inthe form of a circle, the thickness 20 of which being such that themolar teeth 3 are completely enclosed in the sub-volume 18. Duringgeneration of the projection image from the sub-volume 18, summationoccurs along the virtual X-ray imaging direction represented by virtualparallel rays 50.

FIG. 4B shows the sub-volume 18, as shown in FIG. 4A, the onlydifference being that the rays 50 extend in the form of a conical fanstarting from a virtual X-ray source 51, and the basal surface 19 is inthe form of a rectangle.

FIG. 4C shows another alternative embodiment of the sub-volume 18 havinga trapezoidal basal surface 19, the X-ray imaging direction beingrepresented by parallel rays 50.

FIG. 5 is a top view of a schema of a mandibular arch 5 comprising theselected sub-volumes 18, 15, 12, 8 as shown in FIG. 1 and additionalsub-volumes 60 disposed in the X-ray imaging direction 11 oriented atright angles to the course 21 of the mandibular arch 5. Duringgeneration of the individual projection images, summation occurs in themanner shown in FIG. 4A or FIG. 4C along the parallel virtual rays.Unlike FIG. 1, the individual sub-volumes 18, 15, 12, 8 are selected soas to overlap each other so that portions of the mandibular arch 5 areincluded not only in a sub-volume but also in its adjacent sub-volume,so that the same portion of the mandibular arch 5 is visible in theprojection images generated from these adjacent sub-volumes.

FIG. 6 is a schema of a detail of the 3D volume 1 including a singlemolar tooth 3 of the mandibular arch 5. The molar tooth 3 consists of atooth root 70 and a tooth crown 71 provided with a filling 72. The toothroot 70 is disposed in the jawbone 73. The gingiva 74 covers the jawbone73. Secondary caries 76 has developed on the reverse side 75 of thefilling 72 and is to be diagnosed. A sub-volume 77 is defined by theuser or automatically with the assistance of a computer, whichsub-volume 77 comprises the tooth crown 71, while the unwanted object,namely the filling 72, is excluded virtually. The remaining sub-objects,such as the tooth root 70, the gingiva 74, and the jawbone 73, that areof no diagnostic significance are likewise omitted when the sub-volume77 is being defined. The virtual X-ray imaging direction 11 of thevolume 77 extends along a tooth axis in the occlusal direction at rightangles to the occlusal surface 79. The secondary caries 76 can be betterdiagnosed by means of the projection image generated from the sub-volume77 in the X-ray imaging direction 11, due to the fact that the filling72 has been excluded virtually. The filling can alternatively beexcluded by means of a specific function. Such a function is designedsuch that the portions, such as a filling, that are of no significancefor evaluation purposes are made invisible or weakly visible, whilethose portions, such as carious regions, that are significant forevaluation purposes are highlighted in the image.

FIG. 7 is a schema of a detail of the 3D volume 1 as shown in FIG. 6,except that the filling 72 is disposed between two molar teeth 3 and theX-ray imaging direction 11 is oriented in the lingual direction at rightangles to the course 21 of the mandibular arch 5.

FIG. 7 shows a section plane 80 that passes through the center of thetwo molar teeth 3. The sub-volume 81 is formed by the section plane 80and the external contour 82 of the rear surface of the molar teeth 3.This improves the diagnosis of the secondary caries 78 on the reverseside of the filling 72 using the projection image generated from thesub-volume 81 in the X-ray imaging direction 11.

LIST OF REFERENCE NUMERALS OR CHARACTERS

1 3D volume

2 front incisal teeth

3 lateral molar teeth

4 maxillary arch

5 mandibular arch

6 upper jawbone

7 lower jawbone

8 virtual sub-volume

9 basal surface

10 thickness

11 X-ray imaging direction

12 second sub-volume

13 basal surface

14 thickness

15 third sub-volume

16 basal surface

17 thickness

18 fourth sub-volume

19 basal surface

20 thickness

21 course

30 projection image

3 0 arrangement

41 projection image

50 rays

60 further sub-volumes 1-20. (Cancelled).

21. A method for creating a plurality of virtual projection images, themethod comprising: defining a plurality of sub-volumes within an imagevolume represented by volumetric image data, wherein at least onesub-volume includes a sub-object of interest and an unwanted object, andwherein each sub-volume partially overlaps another sub-volume; andsimulating transmission of x-rays through each of the plurality ofsub-volumes using volumetric image data for each sub-volume to generatea plurality of virtual projection images, wherein a simulated x-raytransmission direction is different for each of the plurality ofsub-volumes, and wherein for the at least one sub-volume that includesthe unwanted object, volumetric image data corresponding to the unwantedobject is weighted less than volumetric image data corresponding to thesub-object of interest.
 22. The method as defined in claim 21, whereinthe volumetric image data are x-ray absorption values.
 23. The method asdefined in claim 21, wherein the simulated transmission of x-raysthrough a sub-volume is performed by summing elements of the volumetricimage data for the sub-volume that are successively disposed along thesimulated x-ray transmission direction for the sub-volume.
 24. Themethod as defined in claim 21, further comprising: segmenting thevolumetric image data to identify the sub-object of interest and theunwanted object.
 25. The method as defined in claim 22, wherein thevolumetric image data corresponding to the unwanted object is weightedless by decreasing the magnitude of the x-ray absorption valuescorresponding to the unwanted object.
 26. The method as defined in claim24, wherein an outer contour of the at least one sub-volume, thatincludes the identified sub-object of interest, is defined so as tocorrespond to an outer contour of the identified sub-object of interest.27. The method as defined in claim 21, wherein each of the plurality ofsub-volumes includes a basal surface and the simulated x-raytransmission direction for each sub-volume is substantially orthogonalto the basal surface.
 28. The method as defined in claim 21, furthercomprising: segmenting the volumetric image data to determine positionsand orientations of one or more teeth in a dental arch.
 29. The methodas defined in claim 28, wherein the plurality of sub-volumes are definedbased on the determined positions and orientations of the one or moreteeth in the dental arch such that each sub-volume includes a portion ofat least one tooth.
 30. The method as defined in claim 21, wherein theplurality of virtual projection images are generated according to apredefined schema and displayed, on a display unit, in accordance withthe predefined schema.
 31. The method as defined in claim 21, whereineach sub-volume is defined by (i) defining a basal surface of thesub-volume and (ii) defining a thickness of the sub-volume in adirection orthogonal to the basal surface of the sub-volume, wherein ashape of the basal surface is one of: circular, rectangular, ortrapezoidal.
 32. The method as defined in claim 31, wherein a volume ofspace, defined by an area of a basal surface of the at least onesub-volume and a thickness of the at least one sub-volume in thesimulated x-ray transmission direction, entirely includes the sub-objectof interest.
 33. The method as defined in claim 21, wherein each virtualprojection image represents a total amount of x-ray attenuation througha corresponding sub-volume along the simulated x-ray transmissiondirection.
 34. The method as defined in claim 21, wherein the simulatingtransmission of x-rays through each of the plurality of sub-volumesincludes a simulation of at least one of the following factorsapplicable to generating a conventional intraoral image: X-rayscattering, X-ray detector efficiency, X-ray detector sensitivity, and arelative position between an X-ray detector and an X-ray source.
 35. Themethod as defined in claim 21, wherein the plurality of sub-volumes arearranged in a partially overlapping manner in a direction of a dentalarch.
 36. The method as defined in claim 21, wherein the plurality ofsub-volumes are arranged in a partially overlapping manner so as tofollow a curvature of a dental arch.
 37. The method of claim 21, whereinthe plurality of virtual projection images lie in differenttwo-dimensional flat planes.
 38. A method for creating a plurality ofvirtual projection images, the method comprising: defining a pluralityof sub-volumes within an image volume represented by volumetric imagedata, wherein each sub-volume partially overlaps another sub-volume; andsimulating transmission of x-rays through each of the plurality ofsub-volumes using volumetric image data for each sub-volume to generatea plurality of virtual projection images, wherein a simulated x-raytransmission direction is different for each of the plurality ofsub-volumes.
 39. The method as defined in claim 38, wherein theplurality of sub-volumes are arranged in a partially overlapping mannerin a direction of a dental arch.
 40. The method as defined in claim 38,wherein the plurality of sub-volumes are arranged in a partiallyoverlapping manner so as to follow a curvature of a dental arch.
 41. Themethod as defined in claim 38, wherein the simulating transmission ofx-rays through a sub-volume is performed by summing elements of thevolumetric image data for the sub-volume that are successively disposedalong the simulated x-ray transmission direction for the sub-volume, andwherein the simulated x-ray transmission direction for each sub-volumeis substantially orthogonal to a basal surface of the sub-volume. 42.The method as defined in claim 38, wherein the plurality of virtualprojection images are generated according to a predefined schema anddisplayed, on a display unit, in accordance with the predefined schema.43. The method as defined in claim 38, further comprising: segmentingthe volumetric image data to determine positions and orientations of oneor more teeth in a dental arch.
 44. The method as defined in claim 43,wherein the plurality of sub-volumes are defined based on the determinedpositions and orientations of the one or more teeth in the dental archsuch that each sub-volume includes a portion of at least one tooth.